The main focus of our research has been to understand the biological functions of two highly related signaling molecules, myostatin (MSTN, GDF-8) and GDF-11, which we discovered in a screen for new transforming growth factor- family members. We showed that mice lacking MSTN have dramatic increases in skeletal muscle mass throughout the body, and we and others showed that systemic administration of MSTN inhibitors to mice can promote muscle growth, showing that MSTN acts to limit muscle growth postnatally. Hence, there is extensive interest in targeting MSTN to enhance muscle strength and regeneration, and at least 7 biotechnology and pharmaceutical companies are in phase II/III clinical trials testing myostatin inhibitors in a wide range of disease settings. We showed that GDF-11 plays an important role in axial patterning and in kidney development, and other groups subsequently showed that GDF-11 also regulates the development of the nervous system and pancreas. Gdf11 is expressed widely in adult tissues, and recent studies have suggested that GDF-11 may play an important role in aging. Specifically, circulating GDF-11 levels were reported to decrease with age, and administration of GDF-11 protein to aged mice was reported to reverse age-related tissue dysfunction in the heart, skeletal muscle, and nervous system. Some of these findings have since been challenged by a paper reporting that GDF-11 levels increase with aging and that GDF-11 has a detrimental effect on skeletal muscle regeneration. These findings have raised new questions regarding the roles of MSTN and GDF-11 in adult animals as well as the potential strategies for manipulating the activities of these molecules for clinical applications. A major question in this regard is whether the distinct functions of MSTN and GDF-11 reflect the inherent biochemical differences between these highly related molecules or whether their distinct functions simply reflect their differing pattern of expression. The overall goal of this project is to understand the basis for the distinct functios of these molecules. This project is a continuation of our long-standing effort to understand the mechanisms underlying the regulation and signaling of MSTN and GDF-11 and will utilize the many genetic and biochemical tools that we have developed over many years to target the various components of this regulatory network. The Specific Aims are: to compare the activities of MSTN and GDF-11 in vitro with respect to the roles of known signaling components, including inhibitory binding proteins and receptors; to examine the roles of these binding proteins and receptors in regulating skeletal and cardiac muscle growth and function and skeletal muscle regeneration in vivo; and to carry out gene knock-in experiments in mice in which the MSTN coding sequence is replaced with GDF-11 (and vice versa). The results of these experiments should provide key insights into the mechanistic basis underlying the distinct biological functions that MSTN and GDF-11 carry out in vivo. We believe that these studies will be crucial for developing strategies to exploit the activities of these molecules for therapeutic intervention.